Various techniques are known for testing the authenticity of bank-notes. It should be understood that the term "bank-note" used herein is to be taken to include currency bills and other valuable documents such as traveler's checks, for example. Typically, such techniques involve analyzing the color response of light which may be reflected from a bank-note being tested, to determine whether there is a sufficient degree of correspondence with the color response of a genuine bank-note.
However, the range of colors acceptable for bank-notes is limited. Also, color photocopiers may be used to produce counterfeit bank-notes, and the pigments used in such color photocopiers are liable to change. Furthermore, bank-notes which have been in use for some time may become dirty or stained in various ways, such that their color properties are altered. Such testing techniques, based on color response, therefore have the disadvantage of unduly limited reliability.